1 Answer
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It seems that you are using the dotdeb.org repository for your old PHP, and some other repository for PHP 5.5. The other repository doesn't have php5-apcu package available, therefore it tries to use PHP 5.4 package for it.

The solution is to use dotdeb.org's PHP 5.5 repository to install your PHP 5. This repository should also contain the php5-apcu package you want. To install dotdeb.org's PHP 5 repository, add these two lines to your /etc/apt/sources.list: